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Winter Weather Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0742Z May 15, 2019)
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EDT Wed May 15 2019

Valid 12Z Wed May 15 2019 - 12Z Sat May 18 2019

...Western U.S....

As an anomalously deep upper low/trough moves across the region,
winter is forecast to return to portions of the Sierra, Great
Basin and northern Rockies.

Overall, models are in good agreement showing an upper low closing
off over the eastern Pacific Wednesday night before moving inland
near the California-Oregon border on Thursday.  As it moves inland
on Thursday, this anomalously deep system is expected to carve out
500mb heights more than 2 standard deviations below normal across
much of northern and central California.  At the onset of
precipitation, snows levels will be high - initially confining the
threat for heavy accumulations to areas above 8,000 ft.  WPC
guidance for Day 1 (ending 12Z Thursday) indicates a Slight Risk
for accumulations of a foot or more for portions of the central
Sierra above 8,000 ft. 

As the system moves inland, snow levels will drop significantly
along the Sierra into the Great Basin on Thursday.  WPC guidance
on Day 2 (ending 12Z Friday) shows a Slight Risk for accumulations
of a foot or more for areas as low 5,000 ft along the western
slopes of the central and southern Sierra, with a Moderate Risk
for areas above 7,000 ft.  Significant snow accumulations are
possible farther east across the higher elevations of the Great
Basin, with WPC guidance indicating a Slight Risk or greater for
accumulations of 4-inches or more across the central Nevada ranges.

As the upper low/trough pivots to the northeast, snows will
diminish across California and Nevada while developing farther to
the north across portions of the northern Rockies Friday into
early Saturday.   Increasing upper divergence coupled with low
level convergence and southwesterly inflow will support widespread
precipitation across the region.  With snow levels falling as the
upper low lifts north, the potential for locally heavy
accumulations will increase Friday into early Saturday across
portions of the central Idaho, northwestern Wyoming and
southwestern Montana ranges.

The probability of significant icing is less than 10 percent.